This invention relates to lift type, vertical axis windmills. More specifically, the present invention is directed to a lift type, vertical axis windmill having an omnidirectional diffuser including structural, safety, performance, and control features.
U.S. Pat. No. 4,115,027, the specification of which is incorporated herein by this reference, discloses a vertical axis, lift type windmill having an omnidirectional diffuser. Vertical airfoils that provide aerodynamic lift are mounted with struts around a central shaft to form a rotor. Outwardly of the airfoils, an omnidirectional diffuser includes vertical stators, as part of the rotor support structure, having an aerodynamic elongate cross-section with the major dimension thereof extending essentially radially from the central windmill shaft. The diffuser directs the passing current of air into a zone where the airfoils, having their major dimension perpendicular to their supporting struts, produce lift and impart net torque to the rotor. The diffuser thereby makes the windmill self-start in low to moderate winds and makes it more efficient.
The diffuser stators are designed to provide more efficiency, improved by a factor of two when compared to an equivalent straight bladed vertical axis windmill not having a diffuser. The measured average maximum efficiency of the omnidiffuser vertical axis turbine is 52%. The diffuser is beneficial in low to moderate winds where self-starting properties and high efficiencies are needed.
In high winds, where there is excess power available, these performance characteristics of the diffuser are not needed. However, the stators must withstand large structural forces in high winds because of their size if their cord dimension is perpendicular to the wind vector. The structural members supporting the stators must necessarily be heavy to withstand high winds and to comply with construction codes. Thus, construction costs are significantly increased.
Windmills convert kinetic energy of the wind to a form of useful work. Power in the wind is converted to shaft power. Power in the wind is directly proportional to the turbine swept area, air density, and the cube of the wind speed.
Energy produced by a windmill is the integral of instantaneous power in the wind multiplied by the wind turbine efficiency function of windspeed, integrated over time. Energy produced by a windmill over a given time period therefore is directly proportional to its swept area and a function of the turbine's efficiency as it relates to windspeed. Consequently, turbine swept area is the normalizing factor when comparing windmills or design merits of a particular windmill.
For instance, it is beneficial to reduce the structural weight of a windmill per unit of swept rotor area, which translates into a reduced cost per unit of area. The overall guiding principle in windmill design for wind farm facilities that are designed to produce revenues from the sale of energy is to minimize turbine cost and maximize energy output Normalizing these factors using swept rotor area allows for direct comparison of windmill design features of windmills of different types and sizes.
Aerodynamic efficiency of a windmill is such a normalized factor and is based on swept area. Swept area of the vertical axis windmill is the projected area of the cylindrically inscribed rotor onto a vertical flat plate, and is the true measure of a windmill's size. Wind resistance on the stators causes the structural weight per unit of swept area to be large, approximately 30 lbs. of steel per square foot of swept rotor area.
Stopping a windmill rotor in strong winds is a difficult task because power in the wind increases with the cube of the wind velocity. If the windmill rotor shaft is not restrained by a load (a generator for instance), the turbine freewheels. High winds have enormous power that can quickly drive a windmill rotor to very high speed and consequent destruction.
Lift continuously develops on airfoils in a runaway condition and unless the airfoils can be made to eliminate lift in a runaway condition, or unless a drag device counteracts the effect of lift, the rotor will accelerate to destruction. This is a problem with all types of windmills. A device that nullifies the aerodynamic torque of the rotor in a runaway condition, keeping the rotor from speeding beyond a safe limit, or stopping the windmill in high winds by using a mechanical brake of reasonable size, cost and reliability has previously been needed.
Windmill rotor speed must be controlled depending on the energy application. A windmill generating alternating current with a line excited generator runs at nearly constant rotational speed while generating electricity. The rotor freewheels in winds below the generation threshold. Approaching generation wind speeds, the generator must be turned on in such a way that there are no excessive mechanical or electrical surges that would cause early part failure. Electronic devices that turn the generator on gradually to avoid surges are expensive, vulnerable to harsh environmental conditions, and complex. A simple mechanical device would be preferable.
In overspeed and/or at high wind speeds, the rotor must be controlled to regulate speed or to stop the rotor. Electrical mechanical devices are currently used on most windmills, but they are not fail-safe and are not easily maintained by wind farm operators. Simple, fail-safe mechanical devices are preferred by most operators and can potentially cost less.
Each windmill operates most efficiently at an optimum rotor speed. In dimensionless terms, the rotor achieves peak efficiency at a specific so-called blade tip speed ratio. Tip speed ratio for a vertical axis straight blade windmill is the tangential speed of the blade as it moves on its circular path, divided by the freestream wind speed. Efficiency, at tip speed ratios above or below the optimum, drops off.
The most efficient windmill would operate at the optimum tip speed ratio in all winds. That is not realistic, especially with a constant speed AC generator. The rotor must turn at nearly constant speed over the range of its operating wind speeds. Consequently, only one wind speed produces the optimum tip speed ratio.
The windmill therefore operates off-design most of the time and on-design at one wind speed, the design wind speed. The design wind speed is selected generally for most windmill designs to provide the best efficiency of a turbine design as applied to a broad spectrum of wind resources. Higher efficiency could be achieved if the design wind speed could be changed to suit the wind resource. A simple mechanical drive train/generator subsystem is needed in current windmill designs to achieve a resource specific design wind speed.
Windmill transmissions are particularly vulnerable to shock loads caused by highly variable torque inputs to the transmission operating with a constant speed AC generator. Failure of transmission gears continues to be a major maintenance problem. A simple, low cost mechanical coupling is required to damp these shocks.
An omnidirectional diffuser vertical axis windmill will not function unless the airfoil thickness ratio is greater than a certain value. The windmill will not start, nor will it sustain operation, while employing airfoils with thickness ratios normally employed by most windmills. A family of airfoils specifically applicable to the omnidirectional wind turbine is needed.